1. Field of the Invention
The present invention relates to an apparatus for controlling charging of a storage battery, which is in the form of a battery pack consisting of an assembly of sealed-type nickel-metal hydride batteries or the like, particularly of a storage battery which is to be installed in a movable body such as an electric vehicle.
2. Description of the Prior Art
Conventionally, in controlling charging of a secondary battery which is represented by a nickel-cadmium storage battery, attention is paid to characteristics of a storage battery, such that a voltage of the battery rises as a charge process proceeds and lowers after the time when it reaches the peak value at the full charge, and, when a voltage reduction (what is called -.DELTA.V) of a predetermined amount from the peak value is detected, a control such as stop of charge is conducted.
On the other hand, attention is recently given to a sealed-type nickel-metal hydride battery having a capacity which is very much larger than that of a nickel-cadmium storage battery or the like. The sealed-type nickel-metal hydride battery has noticeable properties that a battery temperature rapidly rises in a period when the charge is approximately completed. Therefore, it is a potent control method that a rapid rising of the battery temperature is detected to thereby make a control to stop charging. For example, Japanese patent publication (Kokai) No. HEI 5-111185 discloses a charge control apparatus which detects a rapid temperature rise and makes a judgment that the charge reaches the final stage.
A sealed-type nickel-metal hydride battery is excellent in fundamental properties such as an energy density, an output density and a life cycle and is under development for practical use as a power source of a movable body such as an electric vehicle. When such a battery is to be used in an electric vehicle, a battery capacity and a total voltage are required to be 50 to 120 Ah and about 100 to 350 V, respectively, in order to obtain a predetermined output. In a nickel-metal hydride battery, since an output voltage of one cell which is the minimum unit in a practical use is about 1.2 V, a number of cells are to be connected in series to one another so as to obtain a required total voltage. For example, when 10 cells are connected in series to constitute 1 module and 24 modules are connected in series, a battery pack of 240 cells is formed and a total voltage of 288 V is obtained. When an assembly of such a large number of cells is to be compactly installed in an electric vehicle or the like, the cells are inevitably arranged in jam-packed. In an arrangement in which cells are jammed with no spatial gap between them, however, a heat radiation property is impaired. Then, a countermeasure such as formation of ribs (projections) on an outer face of a cell container is taken so that cells can make contact with each other while allowing air to pass through therebetween.
FIG. 14 is a diagram showing the basic configuration of a forced cooling apparatus for a battery pack 1 which is to be installed in an electric vehicle. In the figure, the battery pack 1 is provided in a portion through which spaces formed by partition walls 11a and 11b in a container 11 are communicated with each other. Air introduced into the container 11 by an intake fan 12 is exhausted to the outside of the container 11 by an exhaust fan 13 in such a manner that air passes through gaps between the cells and the modules inside the battery pack 1. In this way, the battery pack 1 is forcedly cooled.
In a process of charging the battery pack 1 disposed as described above, even when the control of the charge is to be conducted upon detection of a rise change of the battery temperature as described above while maintaining an air amount for cooling at a constant level, it is not easy to accurately detect the rise change. This is caused by a phenomenon that a cooling rate of the battery varies depending on a temperature difference between the cooling air and the battery.
As described above, since a special storage battery or a battery pack in which a very large number of unit batteries (cells) are densely arranged requires the forced air cooling, it is difficult to adequately carry out the charge control even when only a rising change in the battery temperature is detected. An inadequate control of the charge causes problems that undercharge may impair the battery performance, and that a delay of stop of the charge causes the battery to generate a large amount of heat to increase the internal pressure of each cell, resulting in danger of explosion, etc.